Curable coating compositions, especially thermoset coatings, are widely used in the coatings art. They are often used as topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels.
The effect of the coating process on the environment and the effect of the environment on coatings have increasingly shaped the coatings art in the last few decades. The industry has put considerable effort into developing coatings with materials that will be less harmful toward the environment. Examples of coatings that generally contain lower levels of volatile organic compounds include waterborne coatings, powder coatings, and high solids solvent borne coatings.
However, it has been difficult to devise environmentally sensitive coatings that simultaneously provided desirable resistance to environmental degradation and superior finished film performance properties.
For example, color-plus-clear composite coatings require an extremely high degree of clarity and low degree of visual aberrations at the surface of the coating in order to achieve a high distinctness of image (DOI). As a result, these coatings are especially susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be rubbed out.
It is often difficult to predict the degree of resistance to environmental etch that a high gloss or color-plus-clear composite coating will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints do not provide the desired level of resistance to environmental etch when used in high gloss coatings such as the clearcoat of a color-plus-clear composite coating. Many compositions have been proposed for use as the film-forming component of the clearcoat of a color-plus-clear composite coating. Examples that address the problem of environmental etch resistance include carbamate-aminoplast systems, polyurethanes, acid-epoxy systems and the like. However, several of these prior art systems are vulnerable to application problems.
For example, many prior art coating compositions show an unacceptable tendency to form popping defects at commercially mandated film builds in commercial application environments. A “pop” or “popping defect” as used herein refers to the circular defects or holes that are visible only after the curing of the applied coating film. It should be noted that such defects might also be referred to as “gassing defects” with respect to powder coating compositions and applications. These popping defects are believed to result from the upward passage of volatile gases from the curing film. Such gases may be trapped air, volatile solvents or the volatile by-products of crosslinking reactions. The frequency of pop defects may range from the intermittent to large tightly packed masses of hundreds of individual defects. In either case, the presence of these popping or gassing defects often renders the cured surface commercially unacceptable.
Coating compositions used as topcoats are especially vulnerable to this problem due to their higher film build requirements. Clearcoat coating compositions used in automotive OEM applications need particularly good popping resistance because they are especially vulnerable because they typically require at least 2.5 mils/63.5 microns of applied uncured coating film, more often from 2.6 to 6.0 mils/66.0 to 1524 microns of applied uncured clearcoat, preferably from about 3.0 to 4.8 mil/76.2 to 121.9 microns of applied uncured clearcoat, and most preferably from about 3.6 to 4.6 mils/91.4 to 116.8 microns of applied uncured clearcoat. These high levels of applied uncured coating are often necessary to achieve at least 1.3 mils/33.0 microns of cured coating film, more particularly from 1.3 to 3.0 mils/33.0 to 76.2 microns, and most preferably from about 1.3 to 2.0 mils/33.0 to 50.8 microns of cured coating film.
“Pop resistance” or “pop tolerance” as used herein refers to the resistance of an applied coating at increasing film builds to form bubbles or pops at the surface of the film before or during the curing of the applied film. Coating composition having poor pop resistance will often show popping defects at the edges of parts where paint collects in film builds thicker than that normally intended. Coating compositions having the most desirable pop resistance will not form pop defects until applied at film builds generally greater than those encountered in commercial application environments.
Because of the significant losses in time and resources attributable to popping defects, it would be advantageous to provide a method of improving the popping resistance of coating compositions, especially those intended for use in the automotive industry that have desirable etch resistance. At the same time, such method must produce improvements without any decrease in environmental etch resistance or other commercially required performance property.
It would be particularly desirable to provide a method of improving the popping resistance of coating compositions, especially those intended for use as the clearcoat layer in color-plus-clear composite coatings.